example_14-01.cpp

// Example 14-1. Finding contours based on a trackbar’s location; the contours are
// updated whenever the trackbar is moved

#include <opencv2/opencv.hpp>
#include <iostream>

using namespace std;

cv::Mat g_gray, g_binary;
int g_thresh = 100;

void on_trackbar( int, void* ) {
  cv::threshold( g_gray, g_binary, g_thresh, 255, cv::THRESH_BINARY );
  vector< vector< cv::Point> > contours;
  cv::findContours(
    g_binary,
    contours,
    cv::noArray(),
    cv::RETR_LIST,
    cv::CHAIN_APPROX_SIMPLE
  );
  g_binary = cv::Scalar::all(0);

  cv::drawContours( g_binary, contours, -1, cv::Scalar::all(255));
  cv::imshow( "Contours", g_binary );

}

int main( int argc, char** argv ) {
  if( argc != 2 || ( g_gray = cv::imread(argv[1], 0)).empty() ) {
    cout << "\nExample 14-1: Find threshold dependent contours\nUsage:\n" <<argv[0]
         <<" ../fruits.jpg\n" << endl;
    return -1;
  }
  cv::namedWindow( "Contours", 1 );

  cv::createTrackbar(
    "Threshold",
    "Contours",
    &g_thresh,
    255,
    on_trackbar
  );
  on_trackbar(0, 0);

  cv::waitKey();

  return 0;
}